The PI3K/Akt Pathway Regulates Oxygen Metabolism via Pyruvate Dehydrogenase (PDH)-E1α Phosphorylation

Abstract

Inhibition of the PI3K/Akt pathway decreases hypoxia within SQ20B human head and neck cancer xenografts. We set out to understand the molecular mechanism underlying this observation. We measured oxygen consumption using both a Clark electrode and an extracellular flux analyzer. We made these measurements after various pharmacologic and genetic manipulations. Pharmacologic inhibition of the PI3K/mTOR pathway or genetic inhibition of Akt/PI3K decreased the oxygen consumption rate (OCR) in vitro in SQ20B and other cell lines by 30–40%. Pharmacologic inhibition of this pathway increased phosphorylation of the E1α subunit of the pyruvate dehydrogenase (PDH) complex on Ser293, which inhibits activity of this critical gatekeeper of mitochondrial respiration. Expressing wild type PTEN in a doxycycline-inducible manner in a cell line with mutant PTEN led to an increase in PDH-E1α phosphorylation and a decrease in OCR. Pre-treatment of SQ20B cells with dichloroacetate (DCA), which inhibits PDH-E1α phosphorylation by inhibiting dehydrogenase kinases (PDKs), reversed the decrease in OCR in response to PI3K/Akt/mTOR inhibition. Likewise, introduction of exogenous PDH-E1α that contains serine to alanine mutations, which can no longer be regulated by phosphorylation, also blunted the decrease in OCR seen with PI3K/mTOR inhibition. Our findings highlight an association between the PI3K/mTOR pathway and tumor cell oxygen consumption that is regulated in part by PDH phosphorylation. These results have important implications for understanding the effects PI3K pathway activation in tumor metabolism and also in designing cancer therapy trials that use inhibitors of this pathway.

Aglycemia keeps mitochondrial oxidative phosphorylation under hypoxic conditions in HepG2 cells

“…We might speculate that a certain blockage of PDH phosphorylation should lead to the observed phenotype. For example, the blockage of phosphoinositide 3-kinase/ protein kinase B (PI3K/Akt) mTOR pathway led to the decreased PDH phosphorylation (Cerniglia et al 2015). Previously, preferences for OXPHOS at hypoxia has been reported in hypoxic osteoclasts which increased respiration, ATP production and similarly to our OXPHOS (aglycemic) HepG2 cells increased lactate production (Morten et al 2013).…”

Two-Step Reactivation of Dormant Cones in Retinitis Pigmentosa

“…And, knockdown of PDH in Zebrafish led to loss of cone function (Taylor et al 2004). The PDHε subunit is inhibited by Akt-dependent phosphorylation to block mitochondrial metabolism and drive glucose-derived pyruvate into anaerobic metabolism for ATP production in cancer (Chen et al 2008; Hui et al 2008; Cernigila et al 2015). Txnip regulates Pten to control downstream Akt activity, and it is thus required to prevent inhibitory PDH phosphorylation, thereby promoting aerobic mitochondrial-based conversion of glucose-derived pyruvate into acetyl CoA for production of both fatty acids and NADPH (Yoshioka et al 2013; DeBalsi et al 2014) (see Fig.…”

“…We show constitutive Akt activation, which is a hallmark of cancer cells that drives anaerobic metabolism (i.e., the Warburg effect) (Chen et al 2008; Hui et al 2008; Cernigila et al 2015; Warburg 1956), also highlights dormant cones. Glucose induction of thioredoxin-interacting protein (Txnip), the most highly glucose-inducible gene identified to date, regulates Akt activity to divert cells toward aerobic glucose metabolism and fatty acid synthesis (Chen et al 2008; Hui et al 2008; Cerniglia et al 2015). We provide evidence in cones that glucose induction of Txnip is key in regulating Akt activity and directing glucose metabolism toward OS synthesis.…”

Impaired hippocampal glucose metabolism during and after flurothyl‐induced seizures in mice: Reduced phosphorylation coincides with reduced activity of pyruvate dehydrogenase

“…HIF1α is a transcription factor that is regulated by the phosphatidylinositol‐3‐kinase (PI3K)/protein kinase B (AkT)/mammalian target of rapamycin (mTOR) pathway, activated in hypoxic environments, and increases of reactive oxygen species (ROS), and was upregulated in the rat after lithium‐pilocarpine–induced status epilepticus . HIF1α regulates the transcription of several genes including PDK1 , which leads to the phosphorylation and subsequent activation of PDH . HIF1α was also shown to promote glycolysis by increasing hexokinase 2 expression, and conversion of pyruvate to lactate, in addition to inhibiting oxidative phosphorylation .…”

“…31 HIF1a regulates the transcription of several genes including PDK1, which leads to the phosphorylation and subsequent activation of PDH. 26,27 HIF1a was also shown to promote glycolysis by increasing hexokinase 2 expression, and conversion of pyruvate to lactate, in addition to inhibiting oxidative phosphorylation. 32 This is believed to be a protective mechanism in hypoxic conditions, or during events of oxidative stress to reduce the production of reactive oxygen species, while still maintaining ATP production via glycolysis.…”